WASHINGTON (AP) — Investigators dubbed them "the librarians," four Air Force nuclear missile launch officers at the center of a still-unfolding scandal over cheating on proficiency tests.

"They tended to be at the hub" of illicit exchanges of test information, says Adam Lowther, one of seven investigators who dug into details of cheating that has embarrassed the Air Force and on Thursday brought down virtually the entire operational command of the 341st Missile Wing at Malmstrom Air Force Base, Mont.

At least 82 missile launch officers face disciplinary action, but it was the four "librarians" who allegedly facilitated the cheating, in part by transmitting test answers via text message. One text included a photo of a classified test answer, according to Lt. Gen. Stephen Wilson, who announced the probe's findings Thursday.

Wilson said the four junior officers were at "the crux of it," and that three of the four also are accused of illegal drug activity. The rest of the accused either participated in cheating or were aware of it but failed to blow the whistle, Wilson said.

In response, the Air Force fired nine midlevel commanders at Malmstrom and announced it will pursue a range of disciplinary action against the accused 82, possibly to include courts-martial. A 10th commander, the senior officer at the base, resigned and will retire from the Air Force.

Air Force officials called the discipline unprecedented in the history of America's intercontinental ballistic missile force. The Associated Press last year revealed a series of security and other problems in the ICBM force, including a failed safety and security inspection at Malmstrom, where the exam cheating occurred.

Lowther said the investigations team interviewed missile launch officers and others at the Air Force's two other ICBM bases and found no indication of cheating there.

"Folks clearly crossed the line at Malmstrom," Lowther said in a telephone interview. He is a faculty member at the Air Force Research Institute at Maxwell Air Force Base, Ala.

In an emotion-charged resignation letter titled "A Lesson to Remember," Col. Robert Stanley, who commanded the 341st Missile Wing at Malmstrom, lamented that the reputation of the ICBM mission was now "tarnished because of the extraordinarily selfish actions of officers entrusted with the most powerful weapon system ever devised by man."

Stanley, seen as a rising star in the Air Force, had been nominated for promotion to brigadier general just days before the cheating scandal came to light in January. Instead he is retiring, convinced, as he wrote in his farewell letter Thursday, that "we let the American people down on my watch."

Separately, another of the Air Force's nuclear missile units — the 90th Missile Wing at F.E. Warren Air Force Base, Wyo. — announced that it had fired the officer overseeing its missile squadrons. It said Col. Donald Holloway, the operations group commander, was sacked "because of a loss of confidence in his ability to lead."

The 90th Missile Wing offered no further explanation for Holloway's removal and said it "has nothing to do" with the firings announced by the Air Force in Washington.

Together, the extraordinary moves reflect turmoil in a force that remains central to American defense strategy but in some ways has been neglected. The force of 450 Minuteman 3 missiles is primed to unleash nuclear devastation on a moment's notice, capable of obliterating people and places halfway around the globe.

In a bid to correct root causes of the missile corps' failings — including low morale and weak management — the Air Force also announced Thursday a series of new or expanded programs to improve leadership development, to modernize the three ICBM bases and to reinforce "core values," including integrity.

Air Force Secretary Deborah Lee James, the service's top civilian official, told a Pentagon news conference that a thorough review of how testing and training are conducted in the ICBM force has produced numerous avenues for improvements.

"We will be changing rather dramatically how we conduct testing and training going forward," while ensuring that performance standards are kept high, James said. More funds will be invested in refurbishing the underground ICBM launch control centers and making other infrastructure improvements, she added.

Wilson, head of all Air Force nuclear forces as commander of Global Strike Command, said the changes in training and testing will be far-reaching.

"We're not just putting a fresh coat of paint on these problems," he said. "We're taking bold action."

James had promised to hold officers at Malmstrom accountable once the cheating investigation was completed and the scope of the scandal was clear. None of the nine fired commanders was directly involved in the cheating, but each was determined to have failed in his or her leadership responsibilities.

Wilson said investigators determined that the cheating, which officials originally said happened in August or September last year, began as early as November 2011 and continued until November 2013.

A total of 100 missile launch crew members were identified as potentially involved in the cheating, but nine were cleared by investigators. Another nine of the 100 are being handled separately by the Air Force Office of Special Investigation; eight of those nine involve possible criminal charges stemming from the alleged mishandling of classified information.

The cheating involved unauthorized passing of answers to exams designed to test missile launch officers' proficiency in handling "emergency war orders," which are messages involving the targeting and launching of missiles.

Nine key commanders below Stanley were fired, including the commanders of the 341st Wing's three missile squadrons, each of which is responsible for 50 Minuteman 3 nuclear missiles.

Also sacked were the commander and deputy commander of the 341st Operations Group, which oversees all three missile squadrons as well as a helicopter unit and a support squadron responsible for administering monthly proficiency tests to Malmstrom's launch crews and evaluating their performance.

No generals are being punished. Maj. Gen. Michael Carey, who was fired in October as commander of the 20th Air Force, which is responsible for all three 150-missile wings of the ICBM force, is still on duty as a staff officer at Air Force Space Command but has requested retirement; his request is being reviewed.

Carey was fired after a military investigation determined that he had engaged in inappropriate behavior while leading a U.S. government delegation to a nuclear security exercise in Russia last summer. He was replaced by Maj. Gen. Jack Weinstein.

The cheating at Malmstrom was discovered in early January during the course of an unrelated drug investigation that included two launch officers at Malmstrom and others at several other bases. The drug probe is continuing.